1. Field of the Invention
The present invention relates to packaging of silicon-backed light valves and, in particular, to low-stress, silicon-backed light valve package assemblies and methods of manufacturing the same.
2. Description of the Related Art
Conventional liquid crystal displays (LCDs) include two sheets of glass arranged to form a thin cell which is filled with a liquid crystal material. One of the sheets of glass includes transistors and pixel activation plates on its surface that serve to activate the liquid crystal material at discrete pixel locations.
A silicon-backed light valve is essentially a miniaturized LCD with a silicon backplane substituted for one of the glass sheets. The surface of the silicon backplane includes the transistors (typically CMOS-based), pixel activation plates, and drive circuitry (e.g. row and column drivers) required for operation of the silicon-backed light valve. The pixel activation plates also serve as reflector plates that reflect incoming light back towards a viewer during operation of the light valve.
A representative silicon-backed light valve is shown in cross-section in FIG. 1. Silicon-backed light valve 10 includes a silicon backplane 12 and a glass cover plate 14, which are held in a spaced-apart relationship by precision spacers 16. The thin uniform gap (also referred to as the spacing or LCD cell) formed between the silicon backplane 12 and the glass cover plate 14 is filled with liquid crystal material 18. The combination of silicon backplane 12, liquid crystal material 18 and glass cover plate 14 essentially constitutes a miniature reflective mode, liquid crystal display.
Maintaining a thin uniform gap between the silicon backplane 12 and the glass cover plate 14 (and therefore maintaining a uniform thickness of the liquid crystal material 18) is necessary in avoiding the formation of undesirable optical interference fringe patterns in the display. Such fringe patterns, if visible to a user, render the display unfit for commercial sale.
Typical processes for manufacturing package assemblies for silicon-backed light valves involve first attaching the silicon backplane to a substrate, i.e. either a conventional printed circuit board (PCB) or a PCB that is adhered to a second glass plate, with an adhesive material. After attaching bonding wires between the silicon backplane and the PCB substrate, the bonding wires, portions of the PCB substrate and the silicon backplane are covered by an encapsulant layer to provide mechanical and environmental protection.
A drawback of conventional package assemblies is that the PCB substrate, the adhesive material and the encapsulant layer can all contribute to the exertion of undesirable mechanical stresses on the silicon-backed light valve. The process steps employed in assembling the package (e.g. elevated temperatures during curing of the adhesive material and the encapsulant material) also contribute to the creation of undesirable mechanical stresses. Such mechanical stresses distort the thin uniform gap between the silicon backplane and the glass cover plate resulting in optical interference fringe patterns and rejection of the package assembly. Temperature excursions encountered during use can also lead to mechanical stresses on the light valve and undesirable fringe patterns. Adhering a PCB to a second glass plate in an attempt to provide a substrate that maintains a thin uniform gap is not always successful and incurs additional manufacturing cost.
An additional drawback to the use of an encapsulant layer in siliconbacked light valve package assemblies is the relatively large package size required to accommodate the encapsulant layer. Conventional encapsulation processes rely on dispensing an epoxy encapsulant freely over the bonding wires, peripheral surfaces of the silicon backplane and the PCB substrate. Due to the domed, or convex, shape that the dispensed epoxy encapsulant assumes, the encapsulant layer will naturally spread over a relatively large peripheral area before obtaining a height necessary to completely cover and protect the bonding wires. This spread results in an undesirably large package assembly size.
Once a silicon-backed light valve has been packaged, the entire package assembly must be mounted onto the next-level assembly, for example, a dicroic prism assembly, a metal frame or plastic case of a visual display device, or a sub-assembly of a visual display device. A drawback of conventional silicon-backed light valve package assemblies is the lack of an integral means for mounting the light valve package assembly onto the next-level assembly. Providing additional mounting elements increases cost, package complexity and package size.
Thus, there is a need in the art for a low mechanical stress, cost effective, silicon-backed light valve package assembly, and process for manufacturing the same, that eliminates the formation of user-visible optical interference fringe patterns. The package assembly should also be relatively small in size, cost effective, and provide for mounting into a next-level assembly.